1. Technical Field
The present invention relates to a liquid crystal display device and an electronic apparatus having a backlight unit, and more particularly, a liquid crystal display device and an electronic apparatus, which are capable of reducing the number of various wires associated with a light detector and reducing a wire space, when the light detector for detecting light in a liquid crystal display panel is formed and the brightness of an illumination unit is automatically controlled according to the intensity of the environment light detected by the light detector.
2. Related Art
Recently, a liquid crystal display device has been rapidly applied to an information communication apparatus and a general electric apparatus. Since a liquid crystal display panel itself does not emit light, a transmissive type liquid crystal display device including a backlight unit as an illumination unit is widely used, but, in a portable device, a reflective type liquid crystal display device which does not require a backlight unit is widely used in order to reduce power consumption. However, in the reflective type liquid crystal display device, since environment light is used as an illumination unit, an image is unlikely to be viewed where environment light is suppressed or non-existent. Accordingly, a reflective type liquid crystal display device using a front light unit as an illumination unit or a semi-transmissive-type liquid crystal display device using a transmissive property and a reflective property has been developed.
The reflective liquid crystal display device using the front light unit as the illumination unit can display an image by lighting the front light unit at a dark place and display an image using environment light without lighting the front light unit at a bright place. The semi-transmissive-type liquid crystal display device can display an image using a transmission portion of a pixel area by lighting a backlight unit as the illumination unit at a dark place and display an image using environment in a reflection portion without lighting the backlight unit at a bright place. Accordingly, in the reflective type or semi-transmissive type liquid crystal display device, since the illumination unit, such as the front light unit or the backlight unit, does not need to be always lighted, it is possible to significantly reduce power consumption.
In addition, the transmissive type liquid crystal display device may be an image that can be clearly viewed although the brightness of the backlight unit is decreased at the dark place, but is likely to be viewed when the brightness of the backlight unit is low at the bright place.
As described above, various types of liquid crystal display devices are different from one another in visibility of a liquid crystal display screen by the intensity of environment light. Accordingly, a method of providing a light detector in a liquid crystal display device, detecting the brightness of environment light by the output of the light detector, and controlling the brightness of an illumination unit is known (see JP-A-2002-131719 (claims, paragraphs [0010] to [0013], and FIG. 1), JP-A-2003-215534 (claims, paragraphs [0007] to [0019], FIGS. 1 to 3) and JP-A-2004-007237 (claims, paragraphs [0023] to [0028], FIG. 1).
For example, JP-A-2002-131719 discloses a liquid crystal display device in which a thin-film transistor (TFT) for detecting light is formed on a substrate of a liquid crystal display panel as a light detector and a backlight is automatically turned on/off according to the brightness of surrounding environment light using TFT ambient light photo-sensors for detecting light leakage current of the TFT. JP-A-2003-215534 discloses a liquid crystal display device which uses a photodiode as a light detector and supplies current undergone temperature compensation to a light-emitting diode as a backlight unit according to the brightness of surrounding environment light. JP-A-2004-007237 discloses a portable terminal in which a backlight unit or a light-emitting diode used as an operation display unit of an apparatus is used as a light detector and lighting of the backlight unit is controlled on the basis of an electromotive force of the light-emitting diode according to the brightness of surrounding environment light.
Meanwhile, in the case where the brightness of the illumination unit is controlled by the intensity of environment light as described above, for example, when the light may be temporarily shielded by a hand, it is determined that the environment light weakens by mistake and thus malfunctions may be performed. JP-A-2005-121997 (claims, paragraphs [0036] to [0047], and FIGS. 4 and 5) discloses a backlight illumination method of a liquid crystal display device, in which a plurality of light detectors are provided in the liquid crystal display device and a backlight unit illuminates light only when the outputs of the plurality of light detectors are equally changed. JP-A-2007-094097 (claims, paragraphs [0019] to [0021] and [0029] to [0032], and FIGS. 2 and 3) discloses a liquid crystal display device in which a plurality of light detectors are provided and a backlight unit illuminates light only when majority of the light detectors are changed.
In the inventions disclosed in JP-A-2005-121997 and JP-A-2007-094097, analog output type detectors such as photodiodes or phototransistors are used as the plurality of light detectors, the outputs of the plurality of light detectors are operated, and the backlight unit illuminates light on the basis of the operation result. If the outputs of the analog output type light detectors, output voltage values or output current values are directly related to the intensity of environment light, it is possible to easily determine whether or not the intensity of environment light is equal to or larger than a predetermined value.
In the TFT ambient light photo-sensor employed in JP-A-2002-131719, a time until the output voltage of the TFT ambient light photo-sensor becomes a predetermined voltage value is related to the intensity of environment light. If such a TFT ambient light photo-sensor is used, an original digital operation is necessary for detecting the intensity of environment light. The principle of the TFT ambient light photo-sensor and a general detection circuit will be described with reference to the drawings.
FIG. 13 is a view showing an example of a voltage-current curve of a TFT ambient light photo-sensor. FIG. 14 is a circuit diagram of the TFT ambient light photo-sensor. FIG. 15 is a view showing a capacitor voltage-time curve in the circuit diagram shown in FIG. 14 when brightness is different.
The TFT ambient light photo-sensor has the substantially same configuration as a TFT which is used as a switching element of an active matrix type liquid crystal display panel. Accordingly, it is advantageous that the TFT ambient light photo-sensor can be simultaneously formed when the TFT of the active matrix type liquid crystal display panel is formed. In the TFT ambient light photo-sensor LS, dark current slightly flows in a gate-off area when light is shielded and leakage current is increased according to the intensity (brightness) of light when light reaches a channel portion, as shown in FIG. 13.
As shown in FIG. 14, a constant reverse bias voltage (e.g., −10 V) which becomes the gate-off area is applied to a gate electrode GL of the TFT of the light-receiving portion of the TFT ambient light photo-sensor LS and a capacitor C is connected between a drain electrode DL and a source electrode SL in parallel. One end of the capacitor C and the drain electrode DL are connected to a ground potential. In this state, a constant reference voltage Vs (e.g., +2 V) is applied between the capacitor C by turning on a switching element S1 and then the switching element S1 is turned off, the voltage across the capacitor C is decreased with time according to the brightness of surrounding environment light of the TFT ambient light photo-sensor LS as shown in FIG. 15.
Accordingly, in the TFT ambient light photo-sensor LS, a time until the voltage across the capacitor becomes a predetermined voltage V0 after the switching element S1 is turned off is inversely proportional to the intensity of environment light and the voltage across the capacitor C after the elapse of a predetermined time t0 is inversely proportional to the intensity of environment light. Accordingly, if the time until the voltage across the capacitor becomes the predetermined voltage V0 after the switching element S1 is turned off or the voltage across the capacitor C after the elapse of the predetermined time t0 is measured, the intensity of environment light can be obtained.
In general, the intensity of environment light is converted into an analog output voltage by a sampling hold circuit which is synchronized with the ON/OFF of the switching element S1, the analog output voltage is converted into a digital signal by an A/D converter, and the digital signal is operated such that it is determined whether the intensity of environment light is equal to or larger than the predetermined value or not.
In the inventions disclosed in JP-A-2005-121997 and JP-A-2007-094097, although the plurality of light detectors for detecting environment light are provided, the plurality of light detectors cannot be necessarily used in the same environment and the same condition. For example, if such TFT ambient light photo-sensors are used, a reference voltage value for determining the intensity of environment light is required. The reference voltage value may be previously set, but, actually, the reference voltage is detected by shielding some of the TFT ambient light photo-sensors. Since the spectral sensitivity of the TFT ambient light photo-sensor is not necessarily matched to the visibility of human, some of the TFT ambient light photo-sensors are coated with a color filter layer so as to determine environment light, such that the spectral sensitivity of the TFT ambient light photo-sensor is closer to the visibility of human.
However, if a plurality of TFT ambient light photo-sensors having different systems are used, a plurality of wires are used in each system and thus a wire area forming a wire is increased. If the wire area is increased, the wires of the TFT ambient light photo-sensors are formed in a frame area of the liquid crystal display panel and thus the area of the frame area needs to be proportionally increased. However, in a small-sized liquid crystal display device such as a mobile telephone, since the requirement for a narrow frame of a liquid crystal display panel has been increased, the increase in the size of the frame area is not preferable.